US7026653B2 - Semiconductor light emitting devices including current spreading layers - Google Patents

Abstract

III-nitride or III-phosphide light emitting devices include a light emitting region disposed between a p-type region and an n-type region. At least one heavily doped layer is disposed within either the n-type region or the p-type region or both, to provide current spreading.

Description

BACKGROUND

1. Field of Invention

The present invention is directed to semiconductor light emitting devices including heavily doped current spreading layers.

2. Description of Related Art

Semiconductor light emitting devices such as light emitting diodes (LEDs) are among the most efficient light sources currently available. Material systems currently of interest in the manufacture of high brightness LEDs capable of operation across the visible spectrum include group III–V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials; and binary, ternary, and quaternary alloys of gallium, aluminum, indium, and phosphorus, also referred to as III-phosphide materials. Such devices typically have a light emitting or active region sandwiched between a p-doped region and an n-doped region. Often III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates and III-phosphide devices are epitaxially grown on gallium arsenide by metal organic chemical vapor deposition (MOCVD) molecular beam epitaxy (MBE) or other epitaxial techniques.

Devices grown on a conductive substrate often have the contacts formed on opposite sides of the device. Alternatively, on devices grown on poorly conducting substrates, or for optical or electrical reasons, the device may be etched to expose portions of both the n- and p-type regions on the same side of the device. The contacts are then formed on the exposed regions. If the contacts are reflective and light is extracted from the side of the device opposite the contacts, the device is referred to as a flip chip. Since at least one of the contacts on a flip chip device does not directly overlie the active region, if current is not efficiently spread through the p- and n-type regions, device performance may suffer.

SUMMARY

In accordance with embodiments of the invention, III-nitride or III-phosphide light emitting devices include a light emitting region disposed between a p-type region and an n-type region. At least one heavily doped layer is disposed within either the n-type region or the p-type region or both, to provide current spreading.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross sectional view of a III-phosphide flip chip LED according to an embodiment of the invention.

FIG. 1B illustrates a p-contact for the device illustrated inFIG. 1A.

FIG. 1C illustrates an n-contact for the device illustrated inFIG. 1A.

FIG. 2 is an energy band diagram for a portion of an embodiment of the invention.

FIG. 3 is a cross sectional view of a III-nitride flip chip LED according to an embodiment of the invention.

FIGS. 4A and 4B are a plan view and a cross sectional view of a contacting scheme for a large junction flip chip LED.

FIGS. 5A and 5B are a plan view and a cross sectional view of a contact scheme for a small junction flip chip LED.

FIG. 6 is an exploded view of a packaged semiconductor light emitting device.

DETAILED DESCRIPTION

One approach to improving current spreading in III-phosphide devices in particular is to increase the thickness of the epitaxial layers between the contacts. Thick epitaxial layers generally increase the cost of producing a device and the amount of light lost to absorption. In addition, in a flip chip device, the need to etch to expose portions of buried layers in order to form contacts limits the thickness with which certain device layers can be grown.

In accordance with an embodiment of the invention, a III-phosphide flip chip device includes one or more highly doped layers. The highly doped layers spread current laterally within the device without increasing the thickness of the epitaxial layers in the device.FIG. 1A is a cross sectional view of a III-phosphide flip chip LED. The device ofFIG. 1A includes anactive region6 sandwiched between a p-dopedcladding region5 and an n-dopedcladding region7. The wavelength of light emitted by the active region may be controlled by selecting the width and composition of the layers inactive region6, as is known in the art. An example of a suitable active region includes 3 or 4 quantum wells separated by barrier layers. An n-contact layer8 separates n-contact10 from the n-dopedcladding region7. A p-contact9 is formed on a p-doped current spreadinglayer3. Light is extracted from the device through an undoped, transparentGaP window layer1. The table below gives examples of the thickness, composition, and dopant appropriate for each oflayers3,5,6,7, and8.

P-doped current	1–10 micron thick layer of Mg doped GaP
spreading region 3
P-doped cladding	0.5–2 micron thick layer of Mg dopedAlInP
region
5
Quantum wells of active	80–300 angstrom thick layers ofundoped
region
6	InGaP
Barrier layers of active	100–150 angstrom thick layers of undoped
region 6	(AlxGa1−x)0.5In0.5P, x~0.65
N-doped cladding	0.5–2 micron thick layer of Te doped AlInP
region 7
N-dopedcontact region 8	500 angstrom thick layer of Te doped GaInP

The characteristics given below for each layer are examples and are not meant to be limiting. For example, other p- and n-type dopants, such as Zn or Si, may be used. More information on selecting the appropriate characteristics of the layers of the device may be found inchapters 1–3 of Semiconductors and Semimetals, Volume 64, Electroluminescence I, Academic Press, San Francisco, 2000, Gerd Mueller, ed., which is incorporated herein by reference.

P-contact9 and n-contact10 ofFIG. 1A may be rnultilayer structures, as illustrated inFIGS. 1B and 1C.FIG. 1B illustrates an example of a multilayer p-contact9. A layer of Au—Zn alloy9A is formed adjacent to current spreadinglayer3, in order to provide ohmic contact to the semiconductor layer. Au—Zn layer9A may be protected by an optionalguard metal layer9B of, for example, a sandwich of TiW, TiW:N, and TiW. Athick contact layer9C, such as gold, is then formed overguard layer9B. Theohmic layer9A andguard layer9B may cover all or just a portion of thesemiconductor layer3 on which p-contact9 is formed.

A multilayer n-contact10 may have a similar structure, as illustrated inFIG. 1C. A layer of Au—Ge alloy10A is fanned adjacent to contactlayer8, in order to provide ohmic contact to the semiconductor layer. Au—Ge layer10A may be protected by an optionalguard metal layer10B of, for example, a sandwich of TiW, TiW:N, and TiW. A thickreflective layer10C of Au is deposited overlayers10A and10B.Ohmic layer10A is generally not very reflective, and is thus often formed as dots (as inFIG. 1C) or thin stripes that cover a small fraction of thesemiconductor layer8 on which n-contact10 is formed.

Heavily doped layers4 (represented as thick dashed lines inFIG. 1A) may be included in one or more of p-dopedcontact layer3, p-dopedcladding layer5, and n-dopedcladding layer7. Heavily dopedlayers4 are formed in regions of the device that may benefit from additional current spreading. In the device illustrated inFIG. 1A, p-contact9 does not directly overlie the active region, thus current is required to spread from p-contact9 to the active region. Accordingly, the p-type side ofactive region6 may benefit from additional current spreading and may thus include heavily doped layers4. If n-contact10 is a sheet contact, n-contact10 overlies the entire active region and additional current spreading is not required on the n-type side ofactive region6. If n-contact10 includes small regions ofohmic layers10A and a large reflective sheet10G as illustrated inFIG. 1C, current is required to spread fromohmic contact regions10A to the areas ofsemiconductor layer8 underreflective sheet10C without anohmic contact region10A. In such devices, the n-type side ofactive region6 may benefit from additional current spreading and may thus include heavily doped layers4.

Highly dopedlayers4 are doped with the same conductivity type as the region in which they are formed. For example, highlydoped layers4 within n-type regions of the device are n-type, and highlydoped regions4 within p-type regions are p-type. Typically, highlydoped layers4 are doped with the same dopant species as the surrounding region, though this is not required. Highly dopedlayers4 may be doped to a concentration of about 5×10¹⁸to about 1×10¹⁹cm⁻³. In contrast, n-dopedcladding region7, p-dopedcladding region5, and p-doped current spreadinglayer3 are usually doped to a concentration of about 5×10¹⁷to about 1×10¹⁸cm⁻³.

Due to the high dopant concentration, highlydoped layers4 will tend to absorb light. Accordingly, highlydoped layers4 are usually thin, for example between about 10 and 100 nm thick, and positioned as far from the active region of the device as possible. Multiple highlydoped layers4 may be formed in a single region. In such embodiments, the highly doped layers are usually spaced at least 10 nm apart and the total thickness of all the highly doped layers is between about 100 and about 500 nm.

In some embodiments, highlydoped layers4 are the same composition as the regions in which they are disposed. In other embodiments, highlydoped layers4 are quaternary AlInGaP layer having a lower band gap than the regions in which they are disposed, for example (Al_xGa_1-x)_0.5In_0.5P where 0.2<x<0.7. In general, the smaller the band gap of a material, the more highly the material can be doped without sacrificing crystal quality, thus the use of quaternary alloys for heavily dopedlayers4 may permit these layers to be more heavily doped. In addition, the small regions of lower band gap created by the quaternary heavily doped layers may further encourage lateral current spreading by creating vertical potential barriers.

FIG. 2 illustrates a portion of an energy band diagram for an example of a III-phosphide device incorporating heavily doped layers. Each ofcladding layers5 and7 includes four heavily doped layers4. Cladding layers5 and7 each have a total thickness of about one micron. Heavily dopedlayers4 are each about 50 nm thick, and separated by about 100 nm. Because of the absorptive nature of heavily dopedlayers4, these layers are located in the portions ofcladding layers5 and7 furthest fromactive region6. Heavily doped layers are (Al_0.65Ga_0.35)_0.5In_0.5P doped to a concentration of about 5×10¹⁸cm⁻³.

Heavily doped current spreading layers may also be used in III-nitride light emitting devices.FIG. 3 is a cross sectional view of a III-nitride flip chip device including heavily doped layers. In the device ofFIG. 3, an n-contact layer8 is grown over agrowth substrate20 and optional nucleation layers (not shown), followed by an n-type cladding layer7,active region6, p-type cladding layer5, and p-type contact layer3. As in the device ofFIG. 1A, the wavelength of light emitted by the active region may be controlled by selecting the width and composition of the layers inactive region6, as is known in the art. An example of a suitable active region includes 3 or 4 quantum wells separated by barrier layers. N-contact10 is formed on a portion of n-contact layer8 exposed by etching. A p-contact9 is formed on p-dopedcontact layer3. Both n-contact10 and p-contact9 are reflective and light is extracted from the device throughsubstrate20. The table below gives examples of the thickness, composition, and dopant appropriate for each oflayers3,5,6,7, and8.

P-dopedcontact region 3	0.3–0.7 micron thick layer of Mg doped GaN
P-doped cladding	0.05–0.25 micron thick layer of Mg doped
region 5	AlGaN
Quantum wells of active	100–150 angstrom thick layers ofundoped
region
6	InGaN or AlInGaN
Barrier layers of active	50–150 angstrom thick layers ofundoped
region
6	GaN or InGaN
N-doped cladding region	2–6 micron thick layer of Si dopedGaN
7 and contactregion 8

The characteristics given below for each layer are examples and are not meant to be limiting.

As in the embodiment described inFIG. 1A, in the device illustrated inFIG. 3 heavily dopedlayers4 may be formed in one or more of p-dopedcontact layer3, p-dopedcladding region5, n-dopedcladding region7, and n-dopedcontact layer8. In the device illustrated inFIG. 3, p-contact is usually a sheet contact which provides sufficient current spreading on the p-type side ofactive region6. Accordingly, in III-nitride devices, heavily dopedlayers4 are often formed on the n-type side ofactive region6 only. Heavily dopedlayers4 inFIG. 3 may be GaN layers, quaternary AlInGaN layers, or may be layers of the same composition as the region in which they are located. Heavily doped layers may have the same thickness, location relative to the active region, and dopant concentration as described above in the text accompanyingFIG. 1A.

FIGS. 4A and 4B illustrate an arrangement ofcontacts9 and10 for a large junction device (that is, a device having an area greater than about 400×400 μm²) according toFIG. 1A or3.FIG. 4A is a plan view andFIG. 4B is a cross section taken along line DD.Layers19 includelayers1,3,4,5,6,7, and8 ofFIG. 1A or layers20,7,8,6,5,3, and4 ofFIG. 3. The active region is divided into four isolated regions, in order to minimize the distance between the p- and n-contacts. The contact that is deposited on a layer exposed by etching, i.e. p-contact9 in the device ofFIG. 1A, and n-contact10 in the device ofFIG. 3, surrounds and interposes the four regions. N-contacts10 in the device ofFIG. 1A, and p-contacts9 in the device ofFIG. 3 are formed on the four active regions. P- and n-contacts9 and10 are electrically isolated from each other by air or by optional insulatinglayer22. Sixsubmount connections23 and sixteensubmount connections24 are deposited on the p- and n-contacts to form a surface suitable for connecting the device to a submount. The submount is often a silicon integrated circuit attached to the device by solder joints. In such embodiments, the p- and n-submount connections may be, for example, solderable metals. In other embodiments, the device is connected to the submount by gold bonds, cold welding, or thermocompression bonding.

FIGS. 5A and 5B illustrate an arrangement ofcontacts9 and10 for a small junction device (that is, a device having an area less than about 400×400 μm²) according toFIG. 1A or3.FIG. 5A is a plan view andFIG. 5B is a cross section taken along line CC.Layers19 includelayers1,3,4,5,6,7, and8 ofFIG. 1A or layers20,7,8,6,5,3, and4 ofFIG. 3. The device shown inFIGS. 5A and 5B has a single via21 etched down to a layer of the epitaxial structure below the active region. A p-contact9 in the device ofFIG. 1A, and an n-contact10 in the device ofFIG. 3, is deposited in via21. Via21 is located at the center of the device to provide uniform current and light emission. An n-contact10 in the device ofFIG. 1A and a p-contact9 in the device ofFIG. 3 surrounds the via and provides electrical contact to the other side of the active region of the epitaxial structure. The p- and n-contacts are separated and electrically isolated by one or moredielectric layers22, or by air. Twosubmount connections24 and onesubmount connection23 are disposed on p- and n-contacts9 and10.Submount connection23 may be located anywhere within the region surrounded by insulatinglayer22 and need not necessarily be located directly over via21. Similarly,submount connections24 may be located anywhere in the region outside insulatinglayer22. As a result, the connection of the device to a submount is not limited by the shape or placement of p-contact9 and n-contact10.

FIG. 6 is an exploded view of a packaged light emitting device. A heat-sinking slug100 is placed into an insert-molded leadframe. The insert-molded leadframe is, for example, a filledplastic material105 molded around ametal frame106 that provides an electrical path.Slug100 may include anoptional reflector cup102. The light emitting device die104, which may be any of the devices described above, is mounted directly or indirectly via a thermally conductingsubmount103 to slug100. Acover108, which may be an optical lens, may be added.

Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit of the inventive concept described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.

Claims (18)

1. A semiconductor light emitting device comprising:

a light emitting region disposed between a cladding region of first conductivity type and a cladding region of second conductivity type;

a contact region of first conductivity type adjacent to the cladding region of first conductivity type;

a contact region of second conductivity type adjacent to the cladding region of second conductivity type; and

at least one heavily doped layer disposed within the cladding region of first conductivity type, wherein the heavily doped layer is more heavily doped than the cladding region of first conductivity type.

2. The device ofclaim 1 wherein the light emitting region comprises at least one layer of InGaP.

3. The device ofclaim 1 wherein the light emitting region comprises at least one layer of InGaN.

4. The device ofclaim 1 further comprising a plurality of heavily doped layers disposed within the cladding region of first conductivity type.

5. The device ofclaim 4 wherein:

each of the plurality of heavily doped layers is between about 10 nm and about 100 nm thick; and

the plurality of heavily doped layers are separated by at least 10 nm of cladding region of first conductivity type.

6. The device ofclaim 4 wherein a total thickness of the plurality of heavily doped layers is between about 100 nm and about 500 nm.

7. The device ofclaim 1 wherein:

the cladding region of first conductivity type has a dopant concentration between about 5×10¹⁷and about 1×10¹⁸cm⁻³; and

the heavily doped layer has a dopant concentration between about 1×10¹⁸and about 1×10¹⁹cm⁻³.

8. The device ofclaim 1 wherein the heavily doped layer comprises (Al_xGa_1-x)_0.5In_0.5P, where 0<x≧1.

9. The device ofclaim 8 wherein the heavily doped layer comprises (Al_xGa_1-x)_0.5In_0.5P, where 0.2<x<0.7.

10. The device ofclaim 8 wherein the heavily doped layer comprises (Al_0.65Ga_0.35)_0.5In_0.5P.

11. The device ofclaim 1 wherein the heavily doped layer comprises Al_xIn_yGa_zN, where 0<x≦1, 0<y≦1, and 0<z≦1.

12. The device ofclaim 1 wherein the heavily doped layer comprises GaN.

13. The device ofclaim 1 wherein the heavily doped layer is a first heavily doped layer, the device further comprising a second heavily doped layer disposed within the cladding region of second conductivity type, wherein the second heavily doped layer is more heavily doped than the cladding region of second conductivity type.

14. The deviceclaim 1 wherein the heavily doped layer is a first heavily doped layer disposed within the cladding region of first conductivity type, the device further comprising a second heavily doped layer disposed within the contact region of first conductivity type, wherein the second heavily doped layer is more heavily doped than the contact region of first conductivity type.

15. The deviceclaim 1 wherein the heavily doped layer is a first heavily doped layer disposed within the cladding region of first conductivity type, the device further comprising a second heavily doped layer disposed within the contact region of first conductivity type, wherein the second heavily doped layer is more heavily doped than the contact region of first conductivity type.

16. The device ofclaim 1 further comprising:

a first lead electrically connected to the cladding region of first conductivity type;

a second lead electrically connected to the cladding region of second conductivity type; and

a cover disposed over the light emitting region.

17. A semiconductor light emitting device comprising:

a light emitting region disposed between a cladding region of first conductivity type and a cladding region of second conductivity type;

a contact region of first conductivity type adjacent to the cladding region of first conductivity type;

a contact region of second conductivity type adjacent to the cladding region of second conductivity type;

at least one heavily doped layer disposed within the contact region of first conductivity type, wherein the heavily doped layer is more heavily doped than the contact region of first conductivity type;

wherein:

the contact region of first conductivity type is spaced apart from the light emitting region by the cladding region of first conductivity type; and

the contact region of second conductivity type is spaced apart from the light emitting region by the cladding region of second conductivity type.

18. The device ofclaim 17 the heavily doped layer is a first heavily doped layer disposed within the contact region of first conductivity type, the device further comprising a second heavily doped layer disposed within the contact region of second conductivity type, wherein the second heavily doped layer is more heavily doped than the contact region of second conductivity type.

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